1. Field of the Invention
The invention is directed to a method for the operation of a digital, programmable hearing aid having an input transducer for picking up an input signal and converting it into an audio signal, having a signal processing unit for the processing and frequency-dependent amplification of the audio signal, and having an output transducer. The invention is also directed to a digital, programmable hearing aid for implementing the method.
2. Description of the Related Art
Acoustic feedback frequently occurs in hearing aid devices, particularly for hearing aid devices having a high gain. This feedback is expressed in strong, feedback-caused oscillations at a specific frequency (feedback). This “whistling” is usually extremely unpleasant both for the hearing aid user as well as for persons in the immediate proximity.
Feedback can occur when sound that is picked up via the microphone of the hearing aid device, amplified by a signal amplifier and output via the earphone proceeds back to the microphone and is re-amplified. Two further conditions, however, must be met for the typical “whistling”—usually at a dominant frequency—to occur. The “loop amplification” of the system, i.e., the product of the hearing aid gain and the attenuation of the feedback path, must be greater than 1. Additionally, the phase shift of this loop amplification must correspond to an arbitrary, whole multiple of 360°.
The simplest approach for reducing feedback-caused oscillations is the permanent reduction of the hearing aid gain, so that the loop amplification remains below the critical limit even in unfavorable situations. The critical disadvantage of this approach, however, is that the hearing aid gain required given a more pronounced hearing impairment can no longer be achieved as a result of this limitation.
The whistling typical of feedback usually lies at comparatively high frequencies. Hearing aids with a volume adjustment actuatable by the hearing aid user are known in devices such as the “Swing” hearing aid model of Siemens Audiologische Technik GmbH, with which the gain of an audio signal can be varied. With this model, the boosting or lowering of the amplification of the audio signal ensues dependent on the frequency, where nearly the entire transmission range of the hearing aid is uniformly amplified given a low gain and higher frequencies are less amplified than lower frequencies given a high gain. The frequency-dependent amplification based on the measure of the volume control is thereby static.
German patent document DE 196 24 092 A1 discloses an amplifier circuit for analog and digital hearing aids. For better adaptation to the hearing capability of a test subject, the circuit comprises at least two compression circuits as sub-circuits that superimpose differently, and by which a resulting gain characteristic V can be generated at which the compression ratio decreases with an increasing input level either in a lasting fashion or at defined time intervals.
German patent document DE 196 19 312 A1 discloses an amplifier circuit for a hearing aid in which an input signal exhibits a signal level that is divided into individual, frequency band-specific sub-signal paths (channels).
European patent document EP 0 250 679 B1 discloses a hearing aid with a memory for storing coefficients with respect to a filter frequency response.